Ieda, Y. | Search bioRxiv

bioRxiv2025

Actin and myosin dynamics during epithelial remodeling in avian gastrulation

Epithelial remodeling is powered by contractile forces exerted by the actomyosin cytoskeleton. In invertebrates, pulsatile contractile flows of the medio-apical actomyosin cortex have been shown to be critical in promoting junction contraction that ultimately drive cell rearrangements, apical constriction, and cell extrusion. However, how actomyosin dynamics drives epithelial remodeling in amniotes remains poorly understood. In this study, we generated transgenic quail lines reporting actin and myosin and investigate their dynamics in gastrulating embryos. We show that during this process, epithelial remodeling events are closely associated with the contraction of junctional myosin. Although we observe medio-apical contractile flows, those appear to contribute to junction contraction by promoting junctional myosin recruitment. Notably, by characterizing live and apoptotic cell extrusions and their associated actomyosin dynamics, we provide new insights into the cellular processes underlying primitive streak formation and the emergence of germ layers.

developmental biology↗

bioRxiv2025

Live imaging and functional characterization of the avian hypoblast redefine the mechanisms of primitive streak induction

In birds and mammals, the formation of the primitive streak, the hallmark of the primary axis and site of gastrulation, is thought to occur when an anterior displacement of the hypoblast (visceral endoderm in mice) lifts its inhibition on the posterior epiblast, enabling the activation of NODAL signaling. Although the anterior movement of the murine visceral endoderm is well documented, the dynamics of the avian hypoblast remain poorly understood. Here, using live imaging and quantitative image analysis, we find that the hypoblast is mechanically coupled to the epiblast and does not migrate away from its posterior end. Instead, the hypoblast moves and deforms passively, in response to the forces transmitted from the epiblast that shape the primitive streak, after its induction. Furthermore, we show that the posterior hypoblast does not exert an inhibitory effect on the epiblast but instead expresses NODAL, which activates primitive streak formation. NODAL concomitantly regulates gene expression in the hypoblast, patterning it along the anteroposterior axis. Our results thus redefine how the primary axis is established in avians, demonstrating that the displacement of the hypoblast and its concomitant anteroposterior patterning are consequences -- rather than drivers -- of primitive streak induction, downstream of NODAL signaling. Essential summaryNODAL asymmetric expression in the hypoblast activates primitive streak formation in the avian embryo.

developmental biology↗